Question

A student said, "When the Moon goes around the Earth, there is an inward force due to the Moon and an outward force due to centrifugal force, so the net force on the Moon is zero." Give two or more physics reasons why this is wrong.

Short answer

The net force is non-zero.

Step-by-step solution

Definition of centrifugal force

In a circular motion, centrifugal force works along the radius and is directed away from the circle's centre. When measuring in an inertial frame of reference, the force does not exist. It only becomes relevant when we switch from a ground/inertial to a spinning reference frame.

Finding the net force

The net force on an object can be expressed as the sum of two parts and it is equal to the rate of change of the momentum.

The two parts that we are taken about are the parallel rate of change of the momentum $\frac{dp}{dt}$and the perpendicular rate of change of the momentum $\frac{dp}{dt}$.

Write the net force $F_{net}$on the object.

$F_{net}=\frac{dp}{dt}=\frac{dp}{dt}-\frac{dp}{dt}$

Analysing the momentum

The moon experiences an inward gravitational attraction as it revolves around the Earth. Although there is an outward centrifugal force, it is not a true force. on equating them, the resulting will be a non-zero net force on the moon. As a result, the stated assertion is incorrect.

The object is moving in a curve path, and its momentum is constantly changing. Even if the object moves with constant speed, a change of direction indicates that $\frac{dp}{dt}$is non zero. Hence the net force is non zero.